Colorado Parks and Wildlife data also show a statewide deer population decrease since 2005 by about 36 percent. Across the West, deer decreased by at least 10 percent.

"Mule deer are an indicator species. If mule deer herds are in poor health, it probably means the land itself is in poor condition and that a lot of other species are at risk," National Wildlife Federation public lands policy director Kate Zimmerman said. "For example, sage grouse and mule deer occupy the same habitat for part of the year, and both are in trouble.

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"There are a lot of things we can do. We can get better in the types of activities we allow in deer habitat — like oil and gas development. We can get better in the ways we do oil and gas development in order to reduce the impact on deer habitat."

Next month, CPW leaders are staging a summit in Glenwood Springs to work on strategies to try to boost deer.

"We've had a significant decline that is not satisfactory to us and to the public," Bishop said. "We're going to work collaboratively with all of our constituents to increase mule deer numbers as best as we can. ... If our deer continue to decline, it will hurt us financially."

The extent of oil and gas development on public land "is a consideration," he said. "We work closely with energy companies, like we would any entity — work with them on achieving mitigation as best we can when energy impacts occur."

Demand for deer-hunting licenses. (Click to enlarge.) (The Denver Post)

In Colorado, the latest CPW population estimates, provided in response to Denver Post queries, show a statewide decline in mule deer — the main deer in the West — down to 390,600 in 2013 from 614,100 in 2005.

Some of the decline may be because of changes in methods for estimating deer populations.

Across western states, deer decreased by about 10 percent overall between 2003 and 2009, said Arizona-based wildlife biologist Jim Heffelfinger, who chairs the Western Association of Fish and Wildlife Agencies' Mule Deer Working Group, which draws expertise from 23 states and Canadian provinces.

"We certainly cannot have it all. We need to be smart about our wildlife habitat, especially our mule deer habitat and how we manage the population," Heffelfinger said. "There are so many different things that are stressing mule deer around the West. Fire suppression has closed the forest canopies, and that has reduced the amount of shrubs and weeds that deer rely on.

"You don't really like big, catastrophic fires — certainly where human structures are damaged. But we really need to open up the canopies for deer."

Conservationists pointed to plans for increased oil and gas development on habitat of the already-declining White River herd in northwestern Colorado. Bureau of Land Management officials said they anticipate 15,000 wells will be drilled over the next 20 years on these 1.5 million acres in northwestern Colorado, located between Meeker and Rangely, south of the Yampa River and north of the Colorado River.

That land serves as habitat for other big game, too — and simultaneously holds significant energy resources extractable using the latest technology.

Conservationists urge better spacing of oil and gas facilities, consolidation of facilities, limited construction of roads and adjustments to make sure roads and pipelines don't cut across migration routes.

BLM officials point out that the rights to develop oil and gas in that area already are leased. The federal land managers are developing a plan to manage environmental impact, BLM spokesman Dave Boyd said.

"We're looking at what can be done to mitigate oil and gas impacts," he said. "Should it turn out that it would be more than 15,000 wells, then we would need to do another environmental impact statement.

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